That MT-90 should work as well as mineral oil- - - -the difference in mineral oil and "multi-purpose" gear lube is the extreme pressure additives that are needed for hypoid rear end gears that's in the multi purpose lube. "MT" probably stands for "Manual Transmission" which is just a fancy way of suggesting it's straight mineral oil. Sometimes just letting the clutch pedal out momentarily while in neutral on an upshift will take some of the strain off a weak synchronizer until you get a chance to dig into the box.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!